如何在Swift中只將數組的一部分加起來？

Question

我目前正在試圖在Swift中構建我的第一個應用程序，並且正在尋找相對於當前月份的數組的一部分的總和。這裏是我的代碼：

struct Hour { 
    var date: String? 
    var time: String? 

    init(date: String?, time: String?) { 
     self.date = date 
     self.time = time 
    } 
} 


let hoursData = [ 
    Hour(date: "Nov 29, 2015", time: "7"), 
    Hour(date: "Dec 12, 2015", time: "7"), 
    Hour(date: "Dec 14, 2015", time: "7"), 
    Hour(date: "Dec 25, 2015", time: "7") ] 

我不知道我怎麼會去有關使其中包含time數據的當月總和變量？或者這個問題的任何一個月？任何幫助你們可以給予真正的讚賞。

Answer 1

我可能會做這樣的事情：

// get prefix for current month 

let formatter = NSDateFormatter() 
formatter.dateFormat = "MMM" 
let monthString = formatter.stringFromDate(NSDate()) 

// now add up the time values for that month 

let results = hoursData.filter { $0.date?.hasPrefix(monthString) ?? false } 
    .reduce(0) { $0 + (Int($1.time ?? "0") ?? 0) } 

或者，如果你想添加爲一年一檢查，太：

let formatter = NSDateFormatter() 
formatter.dateFormat = "MMM" 
let monthString = formatter.stringFromDate(NSDate()) 

formatter.dateFormat = "yyyy" 
let yearString = formatter.stringFromDate(NSDate()) 

let results = hoursData.filter { $0.date?.hasPrefix(monthString) ?? false && $0.date?.hasSuffix(yearString) ?? false } 
    .reduce(0) { $0 + (Int($1.time ?? "0") ?? 0) } 

print(results) 

注意，在以上兩者，因爲您使用的是可選項，我使用??來處理值是nil（以及time中是否有非數字字符串）。

---

就個人而言，我建議使用Hour和NSDateFloat而非String，除非你真的需要他們不要使用自選，並使用let代替var：

struct Hour { 
    let date: NSDate 
    let time: Float 

    init(dateString: String, time: Float) { 
     let formatter = NSDateFormatter() 
     formatter.dateFormat = "MMM, d, y" 
     self.date = formatter.dateFromString(dateString)! 
     self.time = time 
    } 
} 

然後代碼變得：

let hoursData = [ 
    Hour(dateString: "Nov 29, 2015", time: 7), 
    Hour(dateString: "Dec 12, 2015", time: 7), 
    Hour(dateString: "Dec 14, 2015", time: 7), 
    Hour(dateString: "Dec 25, 2015", time: 7), 
    Hour(dateString: "Dec 25, 2017", time: 7) 
] 

let calendar = NSCalendar.currentCalendar() 
let currentComponents = calendar.components([.Month, .Year], fromDate: NSDate()) 

let results = hoursData.filter { 
    let components = calendar.components([.Month, .Year], fromDate: $0.date) 
    return components.month == currentComponents.month && components.year == currentComponents.year 
}.reduce(0) { return $0 + $1.time } 

print(results) 

還有進一步優化可能性e（例如不會重複實例化NSDateFormatter），但它說明了使用NSDate對象可讓您使用日曆計算而不是查找子字符串的想法。

Answer 2

你也可以做到這一點功能

let sum = hoursData 
    .filter { $0.date?.hasPrefix("Dec") ?? false } // Only get dates in the correct month 
    .flatMap { $0.time }       // Map to an array of non nil times 
    .flatMap { Int($0) }       // Convert strings to ints 
    .reduce(0) { $0 + $1 }       // Sum up the times 

這可以是簡單了很多，如果你存儲非可選Ints，而不是可選Strings。或者你可以使用NSDateComponents。

Answer 3

首先我會重寫你的結構，
這兩個屬性都可以是不可變的。

而不是爲日期的字符串，我使用的NSDate，和雙爲持續時間

struct Hour { 
    let date: NSDate 
    let duration: Double 

    init(date: NSDate, duration: Double) { 
     self.date = date 
     self.duration = duration 
    } 
} 

我創建的時間像

let hoursData = [ 
    Hour(date: { let c = NSDateComponents(); c.day = 29; c.month = 11; c.year = 2015; return NSCalendar.currentCalendar().dateFromComponents(c)!}(), duration: 7), 
    Hour(date: { let c = NSDateComponents(); c.day = 12; c.month = 12; c.year = 2015; return NSCalendar.currentCalendar().dateFromComponents(c)!}(), duration: 7), 
    Hour(date: { let c = NSDateComponents(); c.day = 14; c.month = 12; c.year = 2015; return NSCalendar.currentCalendar().dateFromComponents(c)!}(), duration: 7), 
    Hour(date: { let c = NSDateComponents(); c.day = 15; c.month = 12; c.year = 2015; return NSCalendar.currentCalendar().dateFromComponents(c)!}(), duration: 7)] 

，如果你想知道這個語法：我使用隱式未命名的閉包創建NSDate參數

現在我篩選一個月和一年使用reduce方法來總結過濾對象

let today = NSDate() 
let totalDuration = hoursData.filter{ 
    let objectsComps = NSCalendar.currentCalendar().components([.Month, .Year], fromDate: $0.date) 
    let todaysComps = NSCalendar.currentCalendar().components([.Month, .Year], fromDate: today) 
    return objectsComps.month == todaysComps.month && objectsComps.year == todaysComps.year 
}.reduce(0) { (duration, hour) -> Double in 
    duration + hour.duration 
} 

---

雖然這是一個很好的答案，我想指出的是，羅布的回答有一個小缺陷：結構Hour隱藏它有日期字符串必須是某種格式的依賴。這違反了Dependency Inversion Principle,D的SOLID Principles。但這很容易解決。

只要在創建Hour實例時通過日期格式化程序。

struct Hour { 
    let date: NSDate 
    let duration: Double 

    init(date: NSDate, duration: Double) { 
     self.date = date 
     self.duration = duration 
    } 

    init (dateFormatter:NSDateFormatter, dateString:String, duration:Double) { 
     self.init(date: dateFormatter.dateFromString(dateString)!, duration:duration) 
    } 
} 

let dateFormatter = NSDateFormatter() 
dateFormatter.dateFormat = "MMM d, y" 

let hoursData = [ 
    Hour(dateFormatter: dateFormatter, dateString: "Nov 29, 2015", duration: 7), 
    Hour(dateFormatter: dateFormatter, dateString: "Dec 12, 2015", duration: 7), 
    Hour(dateFormatter: dateFormatter, dateString: "Dec 14, 2015", duration: 7), 
    Hour(dateFormatter: dateFormatter, dateString: "Dec 25, 2015", duration: 7), 
    Hour(dateFormatter: dateFormatter, dateString: "Dec 25, 2017", duration: 7) 
] 

現在誰曾經使用Hour可以定義爲所需要的格式，可能會在本地化的應用程序很有幫助。

過濾和減少保持不變。

---

但現在我們有了一個新的問題：NSDateformatter'sdateFromString()可能返回零。目前我們用!強制解包，但這可能在生產應用中很糟糕。

我們應該允許適當的錯誤處理，通過允許方便初始化拋出錯誤

enum HourError: ErrorType { 
    case InvalidDate 
} 

struct Hour { 

    let date: NSDate 
    let duration: Double 

    init(date: NSDate, duration: Double) { 
     self.date = date 
     self.duration = duration 
    } 

    init (dateFormatter:NSDateFormatter, dateString:String, duration:Double) throws { 
     let date = dateFormatter.dateFromString(dateString) 
     guard date != nil else { throw HourError.InvalidDate} 
     self.init(date: date!, duration:duration) 
    } 
} 

，如果我們使用它像

do { 
    let now = NSDate() 
    let dateFormatter = NSDateFormatter() 
    dateFormatter.dateFormat = "MMM d, y" 

    let hoursData = [ 
     try Hour(dateFormatter: dateFormatter, dateString: "Nov 29, 2015", duration: 7), 
     try Hour(dateFormatter: dateFormatter, dateString: "Dec 12, 2015", duration: 7), 
     try Hour(dateFormatter: dateFormatter, dateString: "Dec 14, 2015", duration: 7), 
     try Hour(dateFormatter: dateFormatter, dateString: "Dec 25, 2015", duration: 7), 
/*⛈*/ try Hour(dateFormatter: dateFormatter, dateString: " 25, 2017", duration: 7) 
    ] 



    let totalDuration = hoursData.filter{ 
     let objectsComps = NSCalendar.currentCalendar().components([.Month, .Year], fromDate: $0.date) 
     let todaysComps = NSCalendar.currentCalendar().components([.Month, .Year], fromDate: now) 
     return objectsComps.month == todaysComps.month && objectsComps.year == todaysComps.year 
     }.reduce(0) { 
      $0 + $1.duration 
    } 
    print(totalDuration) 
} catch HourError.InvalidDate{ 
    print("one or more datestrings must be wrong") 
} 

錯誤會被抓住。

---

的full code